This invention relates to an organic thin film EL (Electroluminescent) device which is particularly used as a flat panel light source and a display.
An EL device has attracted much interest because of its application as a selfluminous panel-shaped display device. In particular, the organic thin film EL device uses organic material as luminescent material and generates light by high luminous efficiency and high luminance by supplying a supply voltage lower than 10 (V). The organic thin film EL device has several basic structures. One of those structures comprises an anode layer, a hole injection transport layer, an emitting layer, and a cathode layer which are formed on a substrate in this order. Another structure comprises the anode layer, the emitting layer, an electron injection transport layer, and the cathode layer which are formed on the substrate in this order. Other one structure comprises the anode layer, the hole injection transport layer, the emitting layer, the electron injection transport layer, and the cathode layer which are formed on the substrate in this order.
The conventional organic thin film EL device generally comprises a deposited film of aromatic amine as the hole injection transport layer. In particular, it tends to use 1,1-bis(4-diparatolylaminophenyl) cyclohexane, or N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine because of a merit of hole injection characteristic and excellent flatness of the film. Although the deposited film made from the above-mentioned material has uniformity just after deposition process, cohesion in the deposited film considerably progresses few days after. This fact is disclosed in Proceedings contributed to Japan Society of Applied Physics, spring, 93, 30a-SZK-1 and in Proceedings contributed to Japan Society of Applied Physics, autumn, 93, 29a-ZC-8. For the above-mentioned reason, the organic thin film EL device has degradation. Furthermore, the organic thin film EL device has a low glass transition point within a range from 60.degree. C. to 80.degree. C. at most.
Generally, it is regarded that the degradation of the organic thin film EL device is caused because the aromatic amine has the low glass transition point lower than 100.degree. C. This means that the characteristic of the organic thin film EL device is greatly influenced by the film characteristic of an organic layer. For the reason, it is important for the organic thin film EL device to form the film which has homogeneous film characteristic and excellent adhesiveness of an interface to be contacted with another film.
On the other hand, the method using porphyrin chemical compound in the hole injection layer is disclosed in Japanese Unexamined Patent Publication No. Tokkai-Sho 57-51781 (51781/1982). However, the degradation of the organic thin film EL device is caused by the cohesion and crystallization of the hole injection layer after fabrication of the device. This is because the following reason. If organic low-molecular material is used alone, the crystallization and the cohesion progress as time goes by. As a result, it is hard to form the thin film which has the homogeneous film characteristic and which is stable for a long time.
In order to restrain the degradation of the device by improving the thermal stability of the hole injection transport layer, it can be considered that the hole injection transport layer comprises the film wherein the above-mentioned aromatic amine is dispersed into high polymer molecule binder. The hole injection transport layer can be formed by a wet deposition method such as a spin coat method, a dip coat method, or the like. The characteristic of the organic thin film EL device is reported by Kido et al in Japanese Journal of Applied Physics, Vol. 31, 1992, L 960. As disclosed in the report, although the device emits by the high luminance just after the fabrication of the device, the luminance decreases soon under a continuous emission mode at a constant supply voltage. This is caused by the following fact. Namely, the interface between the anode layer and the hole injection transport layer has degraded adhesiveness. For the reason, the hole injection transport layer peels from the anode layer. As a result, defective electrical contact causes between the anode layer and the hole injection transport layer.
Another example of the organic thin film EL device is disclosed in Japanese Unexamined Patent Publication No. Tokkai-Sho 63-295695 (namely, 295695/1988). In such a device, the hole injection transport layer comprises a porphyrin chemical compound layer having hole injection characteristic and an aromatic tertiary amine layer having the hole injection characteristic in order to improve the stability of the organic thin film EL device by reducing the number of pin holes. However, it is insufficient to improve the stability of the organic thin film EL device. This is because it is hard to realize the prevention of peeling of the hole injection transport layer and the prevention of the crystallization and the cohesion in the hole injection transport layer at the same time.
As obvious from the above description, although the organic thin film EL device has the high luminance characteristic, the lifetime thereof is shorter than that of another type of light emission device. This means that it is hard to put the device to practical use.